1. Field of the Invention
The invention generally relates to a noise reduction system for reducing noise energy generated by a noise generating system, and more particularly, to an adaptive noise reduction system that reduces acoustic energy generated by turbofans used in, for example, jet engines.
2. Description of the Prior Art
Current turbofan jet engines that radiate high noise levels are increasingly under scrutiny by the federal government, and more specifically by the Federal Aviation Administration (FAA), due to increased community concern over the problem of noise pollution. In fact, the FAA has pressured aircraft operators to substantially reduce these noise levels by implementing new regulations (e.g., FAR 36 Stage 3 requirements and future more stringent requirements) which must be complied with by the year 2000.
These new regulations pose a serious problem for aircraft operators, such as, for example, shipping, passenger and military operators, since current noise reduction technology cannot meet many of these new requirements. Also, some technology that can meet the current requirements, only do so at great expense by requiring massive retrofits and reconfiguration of the engine compartment, and in some instances, the aircraft itself These same technologies also increase the weight of the plane which, in turn, reduces fuel efficiency. Thus, because of the expense and other factors associated with meeting the strict FAA guidelines, a significant number of aircraft currently in use will not meet the FAA requirements and will have to be removed from service.
There are currently two available alternative technologies for reducing inlet noise in jet engines. One technology simply employs "liners" on the engine compartment which are internal coatings that absorb acoustic energy at the engine inlet. This technology is very limited in that it does not reduce noise over a large frequency range, but is mainly limited to broadband noise. Also, liners become ineffective with time because of changes in material properties due to accumulation of dirt, dust and liquids in the absorptive material. Also, the sound reduction obtained from liners is limited since the amount of reduction is directly proportional to the amount of surface treatment. Thus, if an operator wants to greatly reduce the noise using the liner, the operator must use more liner material over a larger surface area. This adds to the weight of the aircraft and thus affects the fuel consumption of the aircraft.
Technologies for reducing noise also include active noise cancellation systems which are primarily effective at specific frequencies, i.e. tonal noise. To this end, research conducted by Thomas R. J., Burdisso, R. A., Fuller, C. R., O'Brien, W. F., "Active Control of Fan Noise from a Turbofan Engine," AAIA No. 93-0597. 31.sup.st Aerospace Sciences Meeting & Exhibit, Jan. 11-14, 1993, pp. 1-9, conclusively demonstrates that the periodic whine of turbofan noise (both primary frequency and first harmonic) from a commercial Pratt and Whitney JT15D-1 engine radiated forward from the inlet can be successfully reduced. However, in any practical application, the heavy and expensive compression type acoustic drivers may not be able to withstand the real environment. Also, in future engines, with lower blade passage frequencies, even larger and heavier electronic drivers would have to be used, and the poor reliability of the moving parts would be a problem in commercial engines. In addition, the electrical power requirement to drive these compression drivers would require a dedicated source of electrical power.
Not only is fan noise a problem in existing aircraft engines, it has also been identified as a major technical concern in the development of the next-generation engines. To this end, with rising fuel costs, more fuel-efficient aircraft engines are now of great interest to engine designers. One such engine currently in development is the ultra-high bypass (UHB) engine. Although attractive from the standpoint of fuel efficiency, a major drawback of these UHB engines is the high noise levels associated with these engines. Not only will the introduction of ultra high bypass ratio engines in the future, with the bypass ratios in the range of 10, result in a greater fan noise component, with shorter inlet ducts relative to the size of the fan and for the lower blade passage frequencies expected for these engines, passive acoustic liners will have greater difficulty contributing to fan noise attenuation because liners are less effective as the frequencies decrease and the acoustic wavelength increases.
It would thus be advantageous to provide a low cost system, that can be used in new engines and also added as a retrofit to existing engines, for reducing both broadband noise and tonal components without having limitations on sound reduction which are imposed by surface treatments such as passive liners. This system would preferably reduce the noise components without affecting the performance of the aircraft.